The use of liquid carbon dioxide for producing CO.sub.2 snow and subsequently accelerating it to high speeds for cleaning particles from a substrate is taught by Layden in U.S. Pat. No. 4,962,891. A saturated CO.sub.2 liquid having an entropy below 135 BTU per pound is passed though a nozzle for creating, through adiabatic expansion, a mix of gas and CO.sub.2 snow. A series of chambers and plates are used to enhance the formation of larger droplets of liquid CO.sub.2 that are then converted through adiabatic expansion into solid CO.sub.2 "snow". The walls of the ejection nozzle are suitably tapered at an angle less than 15 degrees so that the intensity or focus of the stream of the solid/gas CO.sub.2 will not be reduced below that which is necessary to clean the workpiece. The nozzle, which may be manufactured of fused silica or quartz, does not utilize any precooling.
Lloyd, in U.S. Pat. No. 5,018,667 at columns 5 and 7, teaches the use of multiple nozzles and tapered concentric orifices for controlling the flow of the CO.sub.2 and snow mixture. These references seek to disperse the snow rather than to focus it after exiting the exhaust nozzle. Lloyd teaches that a small portion of the liquid CO.sub.2 is routed through a pilot orifice and then into an expansion cavity for allowing the liquid CO.sub.2 to flash from the liquid to the solid state, which in turn causes a significant drop in temperature. This cooled mixture of solid, liquid and gas cools the inside surface of the nozzle, which then cools the remainder of the nozzle through conduction. This cooling acts as a constant temperature heat sink that precools the liquid CO.sub.2 as it enters the primary orifices in the body, which in turn enhances the conversion of the main flow of the liquid CO.sub.2 flowing through the primary orifices of the nozzle. No precooling gases are used in the vicinity of the nozzle to improve the flashing conversion of the liquid into the solid phase.
Hayashi, in U.S. Pat. Nos. 4,631,250 and 4,747,421, discloses the use of liquified nitrogen (N.sub.2) for cooling a jacket-type peripheral wall defining a sealed cavity in which a flow of CO.sub.2 gas is introduced under pressure. The cooling produced by the cooled peripheral walls causes the CO.sub.2 to change into snow within the chamber. N.sub.2 gas is introduced into the chamber at high pressure in order to agitate and carry the CO.sub.2 snow from the chamber at high velocity though a jetting nozzle. While liquid N.sub.2 is used for cooling the peripheral walls, the ambient N.sub.2 is used only for agitating and transporting the CO.sub.2 snow from the cooled cavity.
In contrast to these prior art teachings, the present invention utilizes inexpensive components and readily available low pressure air for improving the efficiency of creating CO.sub.2 snow and for improving the coagulation of the CO.sub.2 snow into larger CO.sub.2 snow particles. A supersonic nozzle operated in the overexpanded mode is utilized to focus the CO.sub.2 snow onto the workpiece while reducing the shear noise produced by the pressurized exhaust.